Metrics for Environmental Analysis and Eco Efficiency

Various metrics and methods in engineering design are used to evaluate and measure the different aspects of the environmental impact of industrial activities and services. These metrics differ in terms of approach to manage this complex problem, involving various kinds of interactions. An in-depth analysis of this problems is beyond the scope of the present chapter, which will thus be limited to the classification used by Seager and Theis [18]. Sustainable metrics may be characterized in a classification that includes five broad categories:

1. Financial metrics: estimate environmental impacts or ecosystem services in terms of currency so that they may be compared with monetary transactions or industrial accounts. In practice, monetization may lead to the erroneous assumption that environmental exploitation can be reversible in a manner analogous to pecuniary transactions, even if in many cases ecological systems are damaged beyond recovery. Nevertheless, emission trading already exists. For example, the European Union Greenhouse Gas Emission Trading Scheme involves all the 25-member states of the European Union participating in the scheme. Notably, emissions trading contributes little to solve pollution problems, as groups that do not pollute sell their conservation to the highest polluters. However, as discussed in Section 5.2, monetization of the impact on the eco-system can be used in a different way as an additional cost to that of production so as to incentive companies to produce in a cleaner way, bypassing the excuse of global market competition.

2. Thermodynamic metrics: indicate the resource requirements of industrial activities or services, but usually do not include the specific environmental impacts associated with resource consumption and the export of exogenous material into the environment. Only a few, such as the concept of exergy (available work), attempt to indicate whether the resources consumed were used wisely and efficiently.

3. Environmental (including health and safety) metrics: estimate the potential for creating chemical changes or hazardous conditions in the environment. They may be simple measures of what is released to the environment, without chemical considerations such as pollutant degradation, catalysis or recombination to form new pollutants; or they may include potency factors, such as toxicity, reactivity or rarity, and the fate/transport of the pollutants. Most are directed at specific biological or ecological end points, such as death, cancer or mutation, while others may indicate a loss of environmental quality without suggesting any particular ecological manifestation. It is possible for environmental metrics to be expressed in chemical or thermodynamic units. However, environmental metrics are distinguished from thermodynamics by the fact that these are intended to measure environmental loads or changes rather than resource demands. They are generally measures of the waste created by industrial processes rather than by the use of raw materials.

4. Ecological metrics: attempt to estimate the effects of human intervention on natural systems in ways that are related to living things and ecosystem functions. The rates of species extinction and loss of biodiversity are examples, and may be incorporated in the concept of ecosystem health. Ecological metrics relate to biological processes, whereas environmental metrics relate to chemical or other hazardous conditions. For example, a pollution-free environment does not lead to recovery of a depleted bear population if there is a total absence of quality sites due to of the human pressure.

5. Socio-political metrics: evaluate whether industrial activities are consistent with political or ethical goals.

In addition, there are aggregated metrics that combine features or metrics belonging to various categories, or they may group several metrics that belong to a single category.

An example of this composite index is the Environmental Sustainability Index (ESI), which tracks 21 elements of environmental sustainability, covering natural resource endowments, past and present pollution levels, environmental management efforts, contributions to protection of the global commons, and a society's capacity to improve its environmental performance over time [19]. The ESI was developed to evaluate environmental sustainability relative to the paths of other countries. Owing to a shift in focus by the teams developing the ESI, a new index was developed, the Environmental Performance Index (EPI), which uses outcome-oriented indicators, working as a benchmark index that can be more easily used by policy makers, environmental scientists, advocates and the general public [20]. The EPI focuses on two overarching environmental objectives: (i) reducing environmental stresses to human health and (ii) promoting ecosystem vitality and sound natural resource management. The two overarching objectives are gauged using 25 performance indicators tracked in six well-established policy categories, which are then combined to create a final score. Figure 5.1 gives the scheme of how the EPI is constructed.

For each indicator, a relevant long-term public health or ecosystem sustainability goal is identified. These targets are drawn from (i) treaties or other internationally agreements, (ii) standards set by international organizations, (iii) leading national regulatory requirements and (iv) prevailing scientific consensus. The indicators serve as a gauge of long-term environmental policy success. For each country and each indicator, a proximity-to-target value is calculated based on the distance from current results to the policy target. Then, giving a weight to each indicator, the overall Environmental Performance Index is calculated.

This method is designed for evaluation of performances of countries. The same approach, but adapting indicators, could be used also to assess the sustainability of chemical processes. Schwarz et al. [21] have proposed the use of sustainability metrics to guide decision-making managers in chemical companies. In fact, they suggested that a management strategy that incorporates eco-efficiency strives to create more value with less impact. They suggested the use of a few basic indicators of sustainability: (i) material intensity, (ii) energy intensity, (iii) water consumption, (iv) toxic emissions and (v) pollutant emissions and (CO2) emissions. They suggested a limited core set of indicators, but the approach can be expanded using complementary metrics.

Each metric is constructed as a ratio, with impact, either resource consumption or pollutant emissions, in the numerator and a representation of output, in physical or financial terms, in the denominator. To calculate the metrics, all impact numerators and output denominators are normalized.





Natural Resources

Ecosystem Vitality



Environmental Burden Disease Approach

Environmental Burden of Disease

Adequate Sanitation

Drinking Water

Indoor Air Pollution

Urban Particulates

Local Ozone

Regional Ozone

Sulfur Dioxide Emissions

Water Quality Index

Water Stress

Conservation Risk Index

Effective Conservation

Critical Habitat Protection


Natural Resources


Environmental Burden of Disease

Adequate Sanitation

Drinking Water

Indoor Air Pollution

Urban Particulates

Local Ozone

Regional Ozone

Sulfur Dioxide Emissions

Water Quality Index

Water Stress

Conservation Risk Index

Effective Conservation

Critical Habitat Protection

Marine protected areas


Growing Stock


Marine Trophic Index

Trawling Intensity

Irrigation Stress

Irrigation Stress

Agricultural Subsidies

Intensive Cropland

Burnt Land Area

Pesticide Regulation

Climate Change


Emissions/electricity generated industrial carbon intensity

Figure 5.1 Scheme of construction of an Environmental Performance Index. Source: Yale Center for Environmental Law and Policy [20].

Metrics are useful decision-support tools for evaluating alternative processes for the manufacture of a given product. Table 5.1 reports an example, taken from Schwarz et al. [21], which illustrates the metrics for two hexamethylenediamine

Table 5.1 Comparing alternative production processes - metrics for hexamethylenediamine production (more favorable metrics are shown in bold). Source: Schwarz et al. [21].



Hydrocyanation of butadiene

Electrohydrodimerization ofacrylonitrile


lb per SVA




kBtu per SVA




lb per SVA




lb per SVA




lb per SVA




lb per SVA



(HMDA) manufacturing processes. The two sets of metrics clearly show the tradeoffs for these processes. The hydrocyanation process has lower energy-intensity and greenhouse-gases metrics, but its material-intensity, water consumption and pollutants metrics are higher than those for the electrohydrodimerization process.

Similarly, metrics provide a means for comparing resource consumption and pollutant emissions for the manufacture ofvarious products. The comparison ofthe metrics for various processes serves to highlight those areas, such as high energy intensity or toxics emissions, that pose potential business risks. An important characteristic of the metrics is that they are stackable - that is, they can be combined (or stacked) to calculate environmental impact per pound of product over the series of processes that comprise a supply chain.

Figure 5.2, also taken from Schwarz et al. [21], illustrates how metrics for ethylene, chlorine, vinyl chloride and poly(vinyl chloride) (PVC) can be stacked to obtain metrics for the production of PVC, beginning with naphtha and brine. The metrics calculated with the mass denominator can be readily combined. Impact per dollar can also be calculated for a supply chain by combining the single values along the chain in

0.474 lb Ethylene (from naphtha)

0.601 lb Chlorine (from brine)

1.013 lb Vinyl Chloride

1 lb Polyvinyl Chloride

0.601 lb Chlorine (from brine)


Material (lb/lb PVC)

Energy (kBtu/lb PVC)

Water (gal/lb PVC)

Toxics (lb/lb PVC)

Pollutants (lb/lb PVC)

C02 (lb/lb PVC)

Ethylene x 0.474







Chlorine x 0.601







Vinyl Chloride x 1.013







Polyvinyl Chloride







PVC Supply Chain






Figure 5.2 Stacking metrics for the PVC supply chain. Source: Schwarz et al. [21].

Figure 5.2 Stacking metrics for the PVC supply chain. Source: Schwarz et al. [21].

the same way that impact per pound of product is stacked. Note, however, that the values were not updated, and may thus actually be different.

The metrics can be integrated with other analysis tools, such as practical minimum-energy requirements, lifecycle inventories and total cost assessment (TCA), to develop integrated decision-support tools that can provide guidance for decision-makers [8].

BASF has also developed the tool of eco-efficiency analysis to address not only strategic issues but also issues posed by the marketplace, politics and research [22, 23]. The major elements of the environmental assessment include primary energy use, raw materials utilization, emissions to all media, toxicity, safety risk and land use. The basic preconditions in eco-efficiency analysis are [22]:

• the concrete (final) customer benefit is at the heart of the analysis;

• all products or processes studied have to meet the same customer benefit;

• the entire life cycle is considered;

• both an ecological and an economic assessment are carried out;

• impact on health and the danger to people is assessed.

Eco-efficiency assessment focuses in principle on the entire life cycle, but then concentrates on specific events in a life cycle where the alternatives under consideration differ. Eco-efficiency analysis includes the cost data as well as the straight life cycle data. Figure 5.3 shows that life cycle assessment is based on the environmental profile, which can be obtained, for example, from data provided by the plants and which includes the path from the cradle to the work-gate. On extending this approach to the entire life cycle, a life cycle assessment is obtained. Adding to these additional assessment criteria again, followed by an economic assessment, then leads to an eco-efficiency analysis (Figure 5.4).

System boundary

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